Request body: reading body buffering in filters. If a filter wants to buffer the request body during reading (for example, to check an external scanner), it can now do so. To make it possible, the code now checks rb->last_saved (introduced in the previous change) along with rb->rest == 0. Since in HTTP/2 this requires flow control to avoid overflowing the request body buffer, so filters which need buffering have to set the rb->filter_need_buffering flag on the first filter call. (Note that each filter is expected to call the next filter, so all filters will be able set the flag if needed.)

--- a/src/http/ngx_http_request.h
+++ b/src/http/ngx_http_request.h
@@ -302,6 +302,8 @@ typedef struct {
     ngx_chain_t                      *busy;
     ngx_http_chunked_t               *chunked;
     ngx_http_client_body_handler_pt   post_handler;
+    unsigned                          filter_need_buffering:1;
+    unsigned                          last_sent:1;
     unsigned                          last_saved:1;
 } ngx_http_request_body_t;